The discrete dislocation method is employed to determine the effect of pre-existing crack size on the behavior of a plastic tensile crack under static loading. The behavior of a tensile crack under cyclic and reverse loading is also determined by the same method. Three important steps in the fatigue crack growth during each cycle are identified. It is concluded that the details of the fatigue crack growth during each cycle of loading cannot be inferred from the fatigue striations observed on the crack surfaces after fracture. The statistical nature of fatigue is explained by the range of pre-existing crack sizes present in a material which may or may not contain a crack of critical size that can propagate under cyclic or reverse loading.